1. Field of the Invention
The present invention relates to an end emission type semiconductor laser diode, and more specifically, it relates to a semiconductor laser diode stably operating with a high output over a long time while suppressing aged deterioration of an emission end resulting from reduction of a catastrophic optical damage level.
2. Description of the Background Art
When the quantity of current injection is increased in an end emission type semiconductor laser diode of the 650 nm band or the 780 nm band in order to increase the light output, the light output is abruptly reduced to stop lasing. This phenomenon, resulting from catastrophic optical damage (hereinafter abbreviated as COD) to an emission end of the semiconductor laser diode, is conceivably caused through the following mechanism: When current is injected into the semiconductor laser diode, non-radiative recombination current flows through high-density surface levels present on the emission end due to the presence of an oxide or the like. Therefore, the carrier concentration is reduced on the emission end as compared with that in the semiconductor laser diode, leading to light absorption. The semiconductor laser diode generates heat by this light absorption to increase the temperature on the emission end, whereby the bandgap energy on the emission end is reduced to further increase light absorption. This positive feedback loop so extremely increases the temperature of the emission end that the emission end is finally melted to stop lasing.
The COD level of a recent high-output semiconductor laser diode can be increased by disordering an emission end through a window structure for increasing bandgap energy of the disordered emission end and reducing the level of light absorption resulting from surface levels. When the semiconductor laser diode having such a window structure is oscillated over a long period, however, the COD level is disadvantageously reduced due to aged deterioration.
In order to obtain a high COD level, therefore, the temperature of the emission end must be reduced by improving heat radiability of the semiconductor laser diode. In this point of view, various proposals have been made in general.
For example, Japanese Patent Laying-Open No. 03-209895 (1991) (Patent Literature 1) describes that a function of an excellent end protective film can be ensured in a semiconductor laser diode of the 780 nm band having an AlxOyNz film (x+y+z=1) formed as a first dielectric film with an AlGaAs active layer by forming a single AlxOyNz layer having a thermal expansion coefficient close to that of AlGaAs forming the active layer while ensuring heat radiability.
Japanese Patent Laying-Open No. 2003-264333 (Patent Literature 2) discloses a technique of solving such a problem that particles easily adhere to an SiN film exposed on a surface to deteriorate chemical resistance by stacking a first dielectric film of Al2O3, a second dielectric film of Si3N4 or AlN and a third dielectric film of Al2O3 with each other thereby covering the surfaces of the Si3N4 or AlN film with the Al2O3 films on the basis of the idea disclosed in Japanese Patent Laying-Open No. 09-326527 (1997) (Patent Literature 3).